The present invention relates to a method for instantaneous processing of glint noise and to a tracking monopulse radar receiver for carrying out this method.
As is known, a monopulse radar is a tracking radar based on the comparison of two or more different antenna diagrams pointing towards the same target. The monopulse radar provides a measurement of the off-axis angle existing between the radar-target axis detected and the axis of the two antenna diagrams compared. There may be one or more measurements, for example in elevation and/or in relative bearing. The reception antenna diagrams are exploited in at least two reception channels designated as the "sum" channel processing a sum signal and at least one "difference" channel processing a difference signal for the reference plane in question (for example elevation or relative bearing).
If the target is not on the antenna axis in the plane considered the two reception channels receive different signals and comparison of the signals received makes it possible to locate the position of the target relative to the antenna axis. Now, comparing signals having different amplitude and phase with each other is not a simple problem and attempts have been made to reduce the problem to one of the following two cases corresponding to two types of monopulse processing:
"phase monopulse" processing, when the signals compared have amplitudes which are as similar as possible, but have variable relative phases;
"amplitude monopulse" processing, when the signals compared have phases, which are as similar as possible, but have variable relative amplitudes.
To simplify the explanation, the following comments maybe related to a plane, but they are also valid in the general case. Suppose that the target is a point target, that is to say consisting of a single reflecting point, and that the point is off the axis of the antenna by an angle .theta..
If the processing is of the second type, namely "amplitude monopulse" processing, the "sum" and "difference" channels receiving signals S and .alpha. respectively. These signals, which are in phase or in phase opposition, satisfy the following vector equation: .DELTA.=g..theta..S, g being a coefficient of proportionality.
By normalizing the off-axis angle .theta. relative to an angle .theta.o for which .vertline..DELTA..vertline.=.vertline.S.vertline., the following equation is obtained: .DELTA.= .circle.H .S in which H is the normalized off-axis angle.
The "primary" angle-error measurement operator will be called .epsilon..sub.0 and is defined in a known manner by the ratio (1) of the scalar product of the sum vector S and of the difference vector .DELTA. to the square of the modulus of the sum vector S. ##EQU2##
For a pin-point target, if there is no disturbing noise the primay angle-error measurement operator .epsilon..sub.0 is equal to H.
If, on the contrary, the target consists of a plurality N of bright points Mi, with i being between 1 and N, the sum vector S and the difference vector .DELTA. sampled can be written as follows: ##EQU3##
In equations (2) and (3), a.sub.i and .phi.i are respectively the amplitude and the phase of the elementary sum signal corresponding to the bright point Mi, and H.sub.i is the normalized off-axis angle of each bright point Mi relative to the antenna axis.
The elementary parameters a.sub.i and H.sub.i only change very slowly with time. However the elementary phase .phi..sub.i changes rapidly. It varies by 2.pi. when the distance of the bright point Mi from the antenna varies by .lambda./2, where .lambda. is the wavelength of the signal transmitted.
The direction given by the primary angle-error measurement operator .epsilon..sub.0, such as defined above by the equation ##EQU4## therefore undergoes fluctuations which can result in aiming at a point located far outside the wing-span E of the target.
This phenomenon which is called "glint", is particularly troublesome whenever it is desired to guide a projectile such as a missile towards a complex target.
Consequently it is necessary to overcome this phenomenon as much as possible and hence to reduce the so-called "glint" noise.
Several methods exist for reducing this glint noise. We mention only two:
The first method consists in isolating the signals from the various bright points of the target using the Doppler effect, since their relative phases change with time. If it is possible the direction of each point relative to the axis of the antenna can be measured separately.
A method of this type is described, in particular, in French Patent FR-A-2,466,025 in the name of the Applicant Company, corresponding to U.S. application Ser. No. 342,809, now abandoned.
The filtering effected in this way introduces a significant delay into the measurement of the angle-error.
Furthermore the frequency of the various signals changing with time makes them difficult to separate.
A second method is described in French Patent FR-A-2,396,311, also in the name of the Applicant Company, and corresponding to U.S. Pat. No. 4,220,953. The second method is based on the principle that the primary angle-error measurement operator .epsilon..sub.0 is "good", that is to say provides a direction near to that of the barycenter of the target, when the modulus of the sum signal S is sufficiently high relative to its mean value .vertline.S.vertline..sub.mean.
The second method has a better performance than the first method based on filtering. Nevertheless, the second method requires significant processing time because it is necessary to wait for the modulus .vertline.S.vertline. of the sum vector to pass through a relative maximum in order to have an angle-error measurement of good quality.
These two methods illustrate the major disadvantage of the methods of the prior art for overcoming the glint phenomenon as much as possible, namely the delay which they introduce during the calculation of the angle-error measurement operator.
The present invention makes it possible to overcome this disadvantage and relates to a method for processing glint noise from a target, which method, when used in radar receiver, makes it possible to obtain the direction of the target with a reduced glint noise, without additional delay.